TOPIC:

Hello,
1980 KZ750 LTD, 4 carb Keihen cv34, 9000 miles, stored till 2010, carbs cleaned in 2010, only run briefly once a year with fresh non-oxy gas every year. Has run perfectly ever since...until this spring:

This spring after getting the bike going again (new gas, new battery) it has refused to rev down quickly. Upon giving it throttle the rpm goes up strong as it should, but upon release the rpm will fade back but only after 2 or 3 seconds. When shifting gears the rpm does not settle down for a smooth shift. In the past it always behaved as it should.
The throttle cable and twist grip are well lubricated and snap back quickly. The throttle linkage on the 4 carbs also snap back into idle position quickly. But the rpm's are still slow to recede.

I'm guessing this is a carburetor related issue but I am hoping for suggestions and guidance. Has anyone experienced this before?

Thanks,
Les

You have an air leak. Most likely location is at the rubber carb boots between the head and carbs. I'd replace those boots and make sure the boots between the carbs and head are sealing air tight. Also make sure there air filter is in place and in good shape.

Some people will tell you to spray stuff on the engine, such as WD-40 or carb spray, and listen for changes in the idle speed as a way of locating air leaks. In my experience this only turns up gross leaks and is an unreliable method though. Feel free to try, but you may get a false negative result.

Nessism is correct; it sounds like your carb holders are leaking. Year ago I had the same symptoms on my KZ650-C1 and the test below revealed the fact that my carb holders needed to be replaced. After replacing the carb holders on my bike the problem totally disappeared.

You should check the carb holders for leaks. They will eventually develop tiny cracks in the mating surfaces between the holders and the cylinder head that can cause them to leak at idle. These cracks cannot be seen with the carb holders in place. I suggest you try the following. Get the engine warmed up and temporarily set the idle as low as possible without killing the engine (lower than 900 rpm if possible). The lower the better because with the idle very low the slides are nearly shut and the vacuum inside the carb holders is at its highest. Then spray carb cleaner around the carb holders, especially where they mate with the cylinder head and where the carbs attach to them. Carb cleaner works best for this test as it doesn't leave an oily mess like WD40 or evaporate so quickly that it is not drawn into the leaks as can happen with propane or starting fluid (ether). If the engine dies or reacts to the carb cleaner you need to replace the carb holders; trying to stop the leaks using silicone or other products is a waste of time as the leaks will come back quickly. Carb holder leak related problems are noticeable at idle but generally don't have much impact when riding because the throttle is open. If you find you need to replace the carb holders soak the screws with Kroil for a couple days before attempting to remove them, and use JIS standard, NOT Phillips, screwdriver bits, and this will greatly improve the odds that you will not break or strip a carb holder fastener as so many have done. Let me know if you need info on JIS bits, I have a good link for making your own at no cost. Ed

Nessism 650ed,
I appreciate your input. Here is what I did:
I first covered the rubber boots between the carbs and motor.
With the motor at a slow idle I sprayed ether on each of the vacuum ports.
2 from the vacuum switch, 1 from the fuel petcock and 1 covered with a plug.
When I sprayed nothing happened.
Then I removed the rag on the rubber boots and sprayed each individually. Every one of the 4 made the motor increase in rpm. So hopefully that pinpoints the problem.
Here is my plan. Replacing all 4 rubber boots will cost $120 plus shipping (do you know of a better price?), so I thought I would try coating the boots with clear silicone to see if that would work. Even if it is only temporary. The retaining clamps come off easily and hopefully I can apply the silicone without having to remove the carburetors. I'll let you know if it works. Thanks again for your expertise. Les

If it was mine I wouldn't bother putting silicone on them; I would just replace them and be done. Ed

$13.81 each
www.partzilla.com/product/kawasaki/16065...bb253cb8e51f6a67c025